1. Field of the Invention
The present invention relates generally to photographic cassettes, and more particularly to a film sheet presence indicating mechanism for an x-ray cassette to indicate the presence or absence of a film sheet in the cassette.
2. Description of the Prior Art
Photographic cassettes having built-in indicators which enable an operator to visually check whether or not the cassette has been loaded with a film sheet are well known in the art. One of such built-in indicators is disclosed in U.S. Pat. No. 4,394,772 which issued to Okamoto et al on July 19, 1983. The built-in indicator comprises a pair of spaced plates, one of which is provided with a solar battery connected to a liquid crystal indicator. When the cassette is empty of film, the plates are in contact with one another causing shorting resulting in a blank indicator face. When the cassette is loaded with a film, the short-circuiting plates are isolated from each other by the film positioned therebetween resulting in the indicator face displaying the word "film".
Also, in U.S. Pat. No. 3,784,835, which issued to Schmidt on Jan. 8, 1974, simple and inexpensive means are disclosed for indicating whether the cassette contains a film sheet. One of the means disclosed in FIG. 4 comprises a window through which a flag carried by a pivotal lever can be seen. If the cassette contains a film sheet, the sheet will bear against the lever showing the flag through the window. Another means disclosed in FIG. 12 comprises a pin having a large head and a reduced shank. The shank extends through an opening in the bottom wall of the cassette. When the cassette is absent of film, the shank is free to ride up and down in the opening, limited only by the engagement of the head against a shoulder formed within the pressure plate. When a film sheet is loaded in the cassette, the leading end of the film sheet will rest on the head and therefore prevent inward movement of the pin. Thus, an operator can easily determine whether the cassette contains a film sheet merely by determining whether or not the pin is free to move in the opening.
A problem with the battery operated film sheet presence indicating mechanism is that the battery is constantly being drained of current while the cassette is unloaded resulting in a short battery life. Also, a dead battery would cause the indicator to indicate that a film is loaded even though the cassette may be unloaded. A possible problem with the pin-type film sheet presence indicator is that light may pass through the opening in the bottom wall of the cassette which may cause film fogging, resulting in the production of an inferior radiograph. A possible problem with both the pin-type and window-flag film sheet presence indicating mechanisms is that end curl in the leading end of the film sheet may cause the leading end to miss the lever or head resulting in the indication of an unloaded cassette, when in actuality, a loaded cassette should be indicated.